In the field of robotics there has been considerable development of robotic arms having a tip following capability. Such arms can carry a workload or tool and can be used for inspection and repair in confined spaces, for instance within a jet engine or the human body.
A major advance in tip following technology for robotic arms is described in our co-pending Patent Application No. WO 0216995. This application discloses a robotic arm comprising a plurality of longitudinal segments, each of which comprises one or more passive links and a control link. Control ropes or cables are provided that terminate at the control link at the end of each segment, so that by varying the length of the ropes, the arm can be caused to bend and adopt various planar or spatial shapes and configurations. This may be done for example by winding each control rope on or off a spindle using a rotary actuator or pulling the rope directly using a linear actuator. The actuators are located at the proximal end of the arm and are controlled for example by a computer control system. The ropes are generally located in the guide holes disposed towards the outer circumference of the arm.
These arms are suitable for a number of operations which may require the work head at the distal end of the arm to be at adapted or changed for the intended purpose, possibly also with appropriate changes to the control means to provide the desired operation of the arm and the work head. Interchangeable work heads are widely used. However these typically involve location of rigid elements and feed through of services, for instance power and data. Certain work heads may also require additional control ropes for motion control of the work head, which ropes must also pass through the arm. This makes exchanging a work head more complicated, such that it would be advantageous to exchange both the arm and the work head together, whilst retaining the same actuators and control and power systems. The exchangeable component at the distal end (ie the arm and work head) tends to be lower cost than the proximal system end. Furthermore other factors such as wear, or sterilisation requirements, or a change in task, may make exchange of the entire arm preferable to exchange of the work head only.
In order to do this, the exchange or release interface must enable mechanical and electrical power and electronic signals to be transferred. Furthermore the exchange process should not be time consuming, and should be straightforward in comparison with the task to be conducted by the arm. In order to provide an arm which is capable of ‘tip following’ along a predefined path in space in which there is little room for variance or deviation from the defined path, it is necessary to maintain the appropriate length of and tension in each control rope. In practice, due to build variance and operational effects, rope length and tension cannot be assumed. It is therefore essential that, when exchanging an arm, the control rope length and tension are managed.
Furthermore the exchange may be required mid-procedure. It will be appreciated that in such circumstances the replacement arm of the same or different design as the original arm should operate in an equivalent manner. When exchanging an arm the operating algorithm may need to be changed. An arm may have different operating characteristics or may be identical except for some specific calibration parameters that are unique to a particular arm.
Furthermore during the exchange process some actions are common to all control ropes, e.g. disengagement, and some actions of the specific to individual control ropes, e.g. tension control. It is advantageous to simplify the release and connect mechanisms, and where possible to use single mechanisms to achieve an action for multiple axes.
There is, therefore, a need for a robotic arm in which the arm is interchangeable upon a given actuator and motor assembly, where differences in hardware or function may be managed with minimal intervention from the operator, and the coordinated exchange of a number of control ropes may ensure that control rope tension and position are maintained or re-established during the process.
U.S. Pat. No. 6,866,671, U.S. Pat. No. 6,331,181 and U.S. Pat. No. 6,491,701 (Tierney) each describe a releasable tool attachment mechanism for a surgical robot. The tool uses wire ropes to transfer mechanical power from actuators through the interface to the various joints located at the tool tip. The interface also allows data to be exchanged bi-laterally. Within the tool four ropes are wound around four capstans and terminated. The releasable coupling is made between a rotating capstan and a rotating motor.
The capstan is on the tool side of the exchange, which increases the cost and bulk of the tool. Also the rope is required to wrap around the capstan which leads to increased wear and reduced control of rope length due to unequal rope tension during wrap and unwrap. Furthermore the capstans are free to rotate. This means that the mechanism can be back driven and that stored energy in the rope may cause the capstan to unwind.
U.S. Pat. No. 7,331,967 (Lee) describes a variation of Tierney in which three rope to rope connections are managed across three rotary couplings. This solution is complex with multiple rope pulleys and complex mechanisms to manage the coupling process.
Grid type end to end connections are shown in U.S. Pat. No. 6,858,005 (Ohline) which present a different technical problem. It would be expected that the actuator associated with each rope will be of larger diameter than the rope. Hence there must be means of fanning out interface connections to the actuators. One method of achieving this is to use pulleys. These pulleys must be located some distance from the release mechanism unless the release junction is able to ride around the pulleys. Wrapping ropes around pulleys will also lead to more rapid rope wear.